The chief spent Monday night in the hospital but was released Tuesday and "sounded good," Russell said.
Willis, who has been with the department since 1977, has recently come under fire from the Augusta Firefighters Association, whose spokesman Charles Masters says certain departmental policies are putting firefighters' lives in danger.
Russell said he expected the chief to be on medical leave for "an unknown period." Willis' wife, Anne, said last week her husband's doctor recommended he take things easy for a while.
Deputy Chief Mike Rogers is serving as interim chief in Willis' absence, while Battalion Chief Wayne Taylor is interim Emergency Management Agency director, a position Willis also held.